The mountain regions around Darguun don't quite seem to match the tone of the campaign - and in any case, having a massive civilised goblinoid nation next door might diminish the impact of the prospect of a goblinoid invasion.
I placed the region across the Endworld Mountains northeast of Valenar, creating a new trade route that traded goods between the elves of Valenar and the halflings of the Talenta plains on one side, and the Q'barran city of Whitecliff on the other. Those mountains are sufficiently untamed that they can easily contain goblinoid tribes, and yet close enough to the Mror Holds to justify having remnants of former Dwarven occupation.
Here is the adjusted background write-up I used to integrate Elsir Vale into that area of Khorvaire.
[sblock]Adventure Background
This adventure takes place in Eberron in the year 998 YK, in an area known as Elsir Vale, a fertile valley that lies amidst the Endworld Mountains, straddling the border between the Talenta Plains and Q'barra, but not formally considered part of either territory.
The Vale is thinly populated, largely by humans but with a mix of other races, and is very much a frontier area. It is roughly 250 miles wide from east to west, by 70 miles north to south.
History of the Vale
The scattered human towns and villages of the area grew up along the Dawn Way, an important east-west travel route which provides trading links from Valenar and the Talenta Plains in the west, to the towns of Whitecliff and Newthrone in the east. Much of the Dawn Way was built by dwarven tribes which inhabited the Endworld Mountains more than a thousand years ago. While the tribes have long since returned to the Ironroot mountains in the north, their roads, bridges, and cisterns remain in use to this day. After the dwarves left, the presence of various monsters and raiders kept traffic along the Dawn Way light for many years; few caravans dared the long and dangerous trek. Few humanoids lived in Elsir Vale during those years – only scattered settlements of Gatekeeper druidic folk who left behind little more than grassy barrows and stone circles on the hilltops.
About five hundred years ago, the nearby city of Rhest came to control the vale and a large swath of land northwest of the Endworld Mountains as well, ruling them as a protectorate of the Kingdom of Galifar. Soldiers from Rhest secured the roads all the way to Dennovar and beyond, creating a safe passage for trade and opening up Q'barra's northwestern border for explorers and prospectors. More and more traders traveled the Dawn Way, and the fiefdom of Rhestilor grew wealthy on the tariffs exacted from the passing merchants. Under the fiefdom's shield, the towns along the Dawn Way – Brindol, Talar, Terrelton, and the rest – grew up from tiny hamlets or lonely soldiers' posts to flourishing human settlements.
The fiefdom of Rhestilor fell victim to political infighting, as the nearby nations of Cyre and Karrnath each vied to have it declared a part of their territory. Factions within the fiefdom itself disagreed violently whether to side with one Nation or the other, or to retain the fiefdom's nominal independence, and it eventually succumbed to a combination of civil strife, monstrous incursions, and magical blights. Almost two hundred years ago, the city of Rhest was burned by a savage horde out of the Wyrmsmoke Mountains. Although the warriors of Rhestilor killed many of the goblins and their kind, the city was abandoned and the already weakened fiefdom broken. The locks and canals surrounding Rhest fell into disrepair, and the Blackfens swallowed the ruined city.
In the years since the fiefdom's fall, the towns of Elsir Vale have come to look after themselves. Most of the local lords, such as Jarmaath of Brindol, still hold titles derived from the old fiefdom of Rhestilor. The region took little part in the Last War, though it served as a pilgrimage route for the New Galifar settlers who chose to create a new life for themselves as subjects of the old Kingdom of Galifar rather than engage in the factionalism of the Nations. The settlements in Q'barra, such as Whitecliff and Newthrone, have stabilised the Dawn Way as a trading route, and done much to ensure the continued prosperity of the Vale. Today, the Vale remains unclaimed by any nation - though both Q'barra and Valenar would like to include the region in their territory, neither nation has, as yet, been prepared to commit the necessary political and physical resources to such a task.
Vault of Vraath Keep
During a recent adventure, you came into possession of an old map in the hoard of a defeated band of kobold brigands. The map shows the location of Vraath Keep, an ancient fortification in the depths of the Witchwood. Traveling to Elsir Vale, you are making for the closest village to the Witchwood, Drellin's Ferry, hoping to learn more about the Keep before you venture into the forest.[/sblock]